The present invention relates to improvements in dryer drums for paper making machines, and more particularly to an improved mechanism for removing the water condensate which forms on the inner surface of the drum shell from condensing steam and which reduces the heat transfer from the steam within the shell to the surface of the shell for drying a traveling fibrous web.
In a drying drum for a paper making machine, the mechanism includes a cylindrical shell with heads that have shafts for rotational mounting of the drum. A steam supply gland is provided to direct heated steam into the interior of the drum and generally dipper tubes or straws are positioned adjacent the inner surface of the drum to remove condensate which forms against the drum. In the formation of a relatively wide web of paper, the length of the drums has to be substantial, and it is essential for uniform drying that the drum have uniform heat along its length. Since the layer of condensate which forms within the drum reduces heat transfer from the steam to the metal of the drum shell, it is essential that this condensate layer be maintained as thin as possible, but more importantly, that it may be maintained of uniform thickness because if nonuniformity exists, different rates of heat transfer will exist along the length of the drum resulting in temperature differences and differences in dryness of the paper web along its width.
It is accordingly an object of the present invention to provide a means for removing a maximum amount of rimming condensate and removing it very uniformly to maintain a uniform thickness layer and to avoid the adverse effects of heat expansion and contraction which change the spacing between the condensate removal means and the inner surface of the drum shell.
Drying drums for paper making machines are often provided internally with narrow circumferential grooves spaced axially along the inner surface of the shell to leave a series of ribs or lands therebetween. These ribs strengthen the relatively thin shell of the dryer drum, and permit the safe use of a significantly thinner shell through which heat transfer is improved with a consequent increase in efficiency of the drying action on the paper or other web. Condensate is removed from within such drying cylinders through one or more header tubes carrying a plurality of dipper pipes or straws extending into the grooves and drawing off the condensate as it collects in such grooves.
It has been found that, with wide machines having axially long drying cylinders, uneven moisture profiles of the dried web are often present, due to an uneven drying effect across the width of the machine. This uneveness is undesirable, particularly in thin webs, such as tissues, and it is believed that it results from uneven extraction of the condensate due to distortion of the dipper-carrying, condensate-removal header pipes. If, as is generally the case, these pipes are of substantial length, extending across substantially the full axial length of the cylinder, there is a tendency for them to distort locally when hot, and their spacing relative to the inner periphery of the cylinder shell to vary across the width of the cylinder. In this way the positioning of the dipper tubes or straws within the grooves varies, and hence condensate removal through these tubes may vary across the cylinder, producing temperature gradients across the smooth outer drying surface of the cylinder. Attempts to minimize this disadvantage by altering the lengths of the dipper tubes or straws has proved unsuccessful as the temperature gradient across the cylinder may vary according to running conditions of the machine.
The present invention seeks to obviate or reduce the aforementioned difficulty by maintaining the setting distance between the end of each dipper and the inner surface of its associated groove in the shell of the dryer thereby to achieve an even depth of condensate layers. This is achieved by splitting the dipper-carrying tubes into several individual sections, each serving a part-only of the condensate removal across the width of the cylinder, whereby any distortion is minimized.
According to the present invention the dipper tubes or straws across the or each condensate collection region within an internally grooved drying cylinder are carried from a plurality of individual short header tubes which together extend across substantially the whole axial width of the cylinder at said collecting region, the individual short header tubes being connected to one or more common header pipes arranged to lead condensate away from the individual short header tubes into a condensate discharge.
Conveniently the short header tubes are slightly staggered across the width of the cylinder at the or each collection region to facilitate mounting of the tubes while still ensuring that each groove across the region has a dipper tube or straw projecting therein.
Preferably the connections between each short header tube and the common header pipe are flexible, and this eliminates or reduces problems resulting from expansion of the tubes and pipes.
The short header tubes are fastened to the shell locally and hence are able to follow changes in the shape of the dryer resulting from the application of a wet sheet thereto which tends to cause distortion of the dryer especially at the ends. With this local fastening it is possible to maintain the same accurate setting distance of the dipper tubes or straws within their respective grooves. An additional advantage stems from the use of separate common header pipes because flooding of the header in the vicinity of the riser tubes is thereby avoided.
By using a plurality of short header tubes overall manufacture is less exacting inasmuch as accurate location of the dipper tubes or straws within the grooves is less difficult with the short sections than with a single long length. Moreover, the use of short sections of header tube enables internal erosion plates to be changed without removing the headers from the shell.
While the above discussion relates primarily to dryer cylinders or drums which have grooves on the inner surface, the concepts of the invention apply equally to drums which have a smooth inner surface inasmuch as the objective must be to maintain uniform heat transfer from the steam to the metal of the drum shell at all axial locations and to maintain this by maintaining a uniform thickness of condensate at all axial locations along the drum length. It is an object of the present invention to provide a condensate removal means which is so constructed that heat expansion and contraction of the parts will not cause axial displacement of the condensate removal means either axially or radially relative to the drum surface.
It is a further object of the invention to provide an improved mechanism which provides dipper straws with open condensate receiving ends adjacent the inner surface of the shell of a dryer drum wherein the straws are individually divided, or divided in groups, which individual straws or groups are rigidly positioned relative to the inner surface of the shell so that heat expansion of the individual straws or individual groups along a relatively long length of dryer drum does not displace the straws or groups of straws so as to change their effective location with respect to their ability to remove condensate.
Other objects, advantages and features, as well as equivalent structures which are intended to be covered herein, will become more apparent with the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments thereof in the specification, claims and drawings, in which: